The solidified crust was harmful to Al-killed steel casting using a basic tundish flux.After sampling from an actual tundish,XRD,SEM and EDX examinations were carried out to analyze the microstructure of solidified crusts.The conclusions were obtained as follows:main crystallization phases existing in tundish crust were Ca12A14O33,Ca2SiO4 and a little spinel;spinel and Ca2SiO4 distributed between the grain boundaries of Ca12A14O33,which increased the connection strength of crystallization phases by pinning grain boundary,density or hardness of solidified crust maybe also significantly increases;when initial composition of tundish flux was located in spinel region of CaO-SiO2-Al2O3-10%MgO phase diagram,it was easier to precipitate spinel from molten slag;three approaches of spinel formation in flux were summarized.When flux was saturated with magnesia on the metal/flux interface,MgO in flux was reduced by dissolved aluminum and then formed spinel.On the interface of steel/refractory,with feasible aluminum content,it was also easy to form spinel,and spinel inclusions will be floated and captured by tundish flux.
WANG De-yongZHANG Zhi-xiangWANG Hui-huaJIANG Mao-fa
To reveal the effects of magnesium on the evolution of oxide and sulphide inclusions in liquid iron,both thermodynamic calculations and deoxidization experiments were carried out.The samples extracted from the liquid iron were polished and analyzed by optical microscopy and scanning electron microscopy.The results showed that magnesium could modify oxide and sulphide inclusions simultaneously.Spherical MgO and irregular spinel inclusions were observed in the samples.The elongated MnS inclusions were replaced by small MgO·MgS or MgO·MgS·MnS complex inclusions,and the sulphides were distributed dispersively.The evolution mechanisms of inclusions were discussed comprehensively,and a proposed model for the formation of oxysulphide was set up.
To extract the valuable elements from the steel slag, a novel approach has been proposed by modification treatment to provide the stronger driving forces and accelerate the reduction. Three types of dephosphorization steel slags were reduced using carbon-saturated iron bath to extract iron and phosphorus simultaneously. During the process of reduction, slag composition, temperature, and original P2O5 content were investigated respectively. Slag modification treatment, adding either silica or alumina to vary the slag composition, was proven to accelerate the reduction of dephosphorization slag. The equilibrium time can be shortened from 60 to 30 min. Slag modification also allowed the reduction reaction to occur at lower temperature. After slag modification, the original P2O5 content in slag presents a slight difference on reduction process. Almost half of the reduced phosphorus was vaporized within 5 and 20 min. As more and more FeO was reduced, CO gas generation decreased, and evaporation amount of phosphorus therefore decreases.
Al-Ti-O inclusions always clog submerged nozzles in Ti-bearing Al-killed steel.A typical synthesized Al2TiO5 inclusion was immersed in a CaO-SiO2-Al2O3 molten slag for different durations at 1823 K.The Al2TiO5 dissolution paths and mechanism were revealed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).Decreased amounts of Ti and Al and increased amounts of Si and Ca at the dissolution boundary prove that inclusion dissolution and slag penetration simultaneously occur.SiO2 diffuses or penetrates the inclusion more quickly than CaO,as indicated by the w(CaO)/w(SiO2) value in the reaction region.A liquid product (containing 0.7-1.2 w(CaO)/w(SiO2),15wt%-20wt% Al2O3,and 5wt%-15wt% TiO2) forms on the inclusion surface when Al2TiO5 is dissolved in the slag.Al2TiO5 initially dissolves faster than the diffusion rate of the liquid product toward the bulk slag.With increasing reaction time,the boundary reaches its largest distance,the Al2TiO5 dissolution rate equals the liquid product diffusion rate,and the dissolution process remains stable until the inclusion is completely dissolved.
